Lapping device



July 15, 1969 R. w. THOMAS ETAL 3,455,067

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United States Patent 01 lice 3,455,067 Patented July 15, 1969 3,455,067 LAPPING DEVICE Robert Warren Thomas, Syracuse, and Edward J. Huebner and Harold Yerger, Rome, N.Y., assignors to the United States of America as represented by the Secretary of the Air Force Filed June 21, 1967, Ser. No. 648,541 Int. Cl. B24b /00, 41/06, 29/00 US. Cl. 51-237 Claims ABSTRACT OF THE DISCLOSURE A lapping device particularly suited for lapping flat surfaces having an adjustable reference plate and a three point micrometer adjustable platform for holding a wafer to be lapped.

The present invention relates generally to lapping devices, and more particularly, to a device adapted for lapping objects having fiat surfaces to within extremely close tolerances.

Lapping may be defined as precision finishing with abrasive. Specifically, the process, which is performed by hand or machine, improves the geometric accuracy of fiat surfaces and holes and refines the surface finish. Lapped surfaces are often required to obtain longer wear of moving parts, to obtain better seals, or to obtain longer life of cutting edges, as in dies.

Modern lapping machines have made possible the quality production of parts, the cost of which would have been prohibited by the old hand-lapping method. Industrial requirements demand lapping machines arranged to use three types of lapping mediums. One uses metal laps and loose abrasive grain mixed with a lubricant and will always find a place in the field of gage manufacture, or for other operations where extreme accuracy is required. Another type of machine uses bonded abrasives for commercial production work. A third and more recent type of machine employs abrasive paper or cloth instead of castiron or bonded abrasive sticks. A very bright finish is obtained and a genuine lapped surface is produced.

In lapping fiat surfaces, which are usually on hardened steel, a cast-iron plate is used as a lap, with a good abrasive. For rough work or blocking down, the lap works better if scored with narrow grooves, both lengthwise and crosswise, thus dividing the plate into small squares. The abrasive is sprinkled loosely on the block, wet with lard oil and the work rubbed on it; care is taken to press hardest on the highest spots. The abrasive and oil get into the grooves and are continually rolling in and out, getting between the plate and the work, and are crushed into the cast-iron, thus charging it thoroughly in a short time.

After blocking down, or if the work has first been ground on a surface grinder, the process is different. A plain plate is used with the best quality of flow of emery as an abrasive. Instead of oil, benzene is used as a lubricant, and the lap should be cleaned often and fresh benzene and emery applied as often as it becomes sticky. The work should be tried from time to time with a straight edge and care taken not to let the emery run in and out from under the work, as this will cause the edges to abrade more than the center and will especially mar the corners.

Some shops grind flat work to within 0.0005 inch of size and lap to within 0.0002 inch on a tight-metal lap charged with flour emery.

In the past it has been difficult to produce a finished surface fiat to a tolerance of better than 0.0002 inch. Such a tolerance is insufiicient to accommodate the fits necessary in the industrial demands of today.

The lapping machine and control system of this invention have solved a major problem in the fabrication of an advanced microelectronic circuit. In this operation, it is necessary to lap a crystalline silicon wafer to extremely close tolerances. The machine makes practical a fabrication process developed commercially, but abandoned because of the mechanical lapping control problems.

It is, therefore, the primary object of this invention to lap flat surfaces. More particularly, it is the object of this invention to produce a finished fiat surface to better than 0.00001 inch over a one-inch diameter wafer with lapping angle adjustments in increments of less than 0.0001 inch.

A further object of this invention is to produce a lapping machine which has little or no wear between the surfaces used to establish the reference plane during the lapping operation.

Another object of this invention is to have a reference plane which is adjustable and which can be positioned exactly parallel to the lapping wheel.

Still another object of this invention is to produce a lapping device which has the ability to adjust the rotating Wafer holder with three degrees of freedom.

It is still another object of this invention to provide a lapping device which is economical to produce and utilizes conventional, currently available components that lend themselves to standard mass production manufacturing techniques.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

In the drawing:

FIGURE 1 is an elevation view of the invention;

FIGURE 2 is a bottom view of the invention; and

FIGURE 3 is a cross sectional view of the invention taken mainly along vertical center line III-III of FIG- URE 1 and diverted to encompass a section through the ball bearing assembly.

The lapping system, as conceived by the inventors, embodies a three point micrometer adjustable reference plate 10 (shown in FIGURE 3) made of steel or other suitable material, and mounted in a first support 37 by means of any suitable mounting means such as annular retaining ring 13 and screws 11. The reference plate 10' can be adjusted parallel to the surface of a rotating lapping wheel 12. These adjustments are made with two micrometers 14 and 16 and a screw 18 (shown in FIGURES 1 and 2) operably connected between the first support 37 and a second support 34. The support 34 along with a third support 36 rigidly support the lapping device above the lapping wheel 12.

The reference plate 10 is hand-lapped on a glass plate using a silicon carbide grit (not shown) until an optically flat surface is obtained. This reference plate 10 is then used as an absolute reference for all future lapping operations It is, of course, possible to obtain a cone shaped lap by adjusting the reference plate 10 out of parallelism with the lapping wheel 12 by means of micrometers 14 and 16 and screw 18.

The substrate mounting assembly 22 comprises of a rotating, three point, micrometer adjustable platform 24 (made of two plates 23 and 25) which is driven from above by a shaft 26 and a pin arrangement (shown in FIGURE 3). The shaft 26 is driven by means of a pulley 30 and belt 32 from a motor (not shown). If desired, the motor may be directly coupled to the shaft.

The pin arrangement comprises of a plurality of pins 29 fixedly mounted on shaft 26 and in inter-engagement with slots 31 of self-aligning ball bearing assembly 38, although any other suitable connection may be used between the shaft 26 and the platform 24.

The pin arrangement allows tilting of the platform 24 about the self-aligning ball bearing assembly 38. The

inner bearing race 40 of the ball bearing assembly is attached to the drive shaft 26 by means of any suitable means such as a washer 42 and a screw 44. The outer bearing race 46 is attached to a drive flange 39 which is secured to the rotating platform 24 by any conventional securing means such as bolt or bolts 48.

The platform 24 is spring loaded against the reference plate by means of spring 50 (shown in FIGURE 3). Spring 50 bears against washer 52 mounted on fixed support 36 and washer 54 mounted on shaft 26 and is held in place by means of bolt 56. The platform 24 rides on three Teflon feet 58 which are adjustable by at least three platform micrometer tilting assemblies 60 (shown in FIG- URE 1). Each micrometer tilting assembly 60 consists of a micrometer 62; a tapered spindle 64 which positions the top of Teflon feet 58 at the desired distance from the platform 24; a spring biased socket support 65 and a socket 68 (shown in FIGURE 3). The micrometer 62 and tapered spindle 64 are designed to give one thousandth vertical travel for every complete revolution of the micrometer. The Teflon feet 58 are designed with balls 66 which fit in sockets 68 to allow vertical adjustment of one reference foot without changing the distance to reference plate 10 of the other two reference points.

Before the wafer 70 is mounted on the wafer holder 20, the flatness of the holder is checked with a Taylor-Hobson-Talysurf with an aboslute accuracy of 0.000001 inch. The wafer 70 is mounted on the wafer holder by shellac or other suitable adhesive. The wafer holder with wafer thereon is inserted into a spring loaded center cylinder 72, while the cylinder is held in the up position by a spring biased pin 74. Once the holder 20 is inserted, it is secured to the cylinder 72 by any conventional means such as a bayonet-type fastening arrangement and given a twist with a mounting wrench (not shown). The pin 74 is then retracted and the holder 20 and wafer 70 are lowered to the lapping wheel 12 (see FIGURE 3 A specially designed Teflon sleeve 76 is used to maintain a zero tolerance press fit between the spring loaded cylinder 72 and the housing 73. The lapping force can be adjusted by changing the spring 78 used within the cylinder 72. In order to remove the cylinder 20 from the substrate assembly 22, a safety pin 80 must first be removed.

A typical operation of the machine will now be described; initially the hand-lapped, optically flat reference plate 10 is inserted into the first support 37 and fastened in place by retaining ring 13 and screws 11. The reference plate 10 is now adjusted parallel to the surface of the rotating lapping wheel 12 by means of micrometers 14, 16 and screw 18. The wafer 70 to be lapped is mounted on a wafer holder 20. The flatness of the holder 20 was previously checked with an absolute accuracy of 0.000001 inch. The wafer holder 20 with wafer 70 thereon is inserted into spring loaded center cylinder 72, while the cylinder is held in the up position by spring biased pin 74. Once the holder 20 is inserted, it is given a twist with a mounting wrench (not shown) to secure the holder 20 to the cylinder 72. The substrate mounting assembly 22, containing the wafer 70 to be lapped, is adjusted by means of the three point micrometer adjustable platform 24. The pin 74 is then retracted and the holder 20' with wafer 70 thereon is lowered to the lappingwheel. If desired the lapping force can be adjusted by changing the spring 78 inside cylinder 72.

Although the invention has been described with reference to a particular embodiment, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.

What is claimed is:

l. A lapping device comprising a first support, a reference plate mounted on a second support, a driven shaft operably positioned through said supports and rotatably mounted on a third support, pin means mounted at one end of said shaft, a substrate mounting assembly operably connected to said pin means and a holder mounted on said substrate mounting assembly for holding a wafer to be lapped.

2. A lapping device as defined in claim 1 wherein said reference plate is adjustable.

3. A lapping device as defined in claim 1 wherein said substrate mounting assembly is adjustable.

4. A lapping device as defined in claim 2 further comprising at least a pair of micrometers and a screw operably connecting said first support to said second support in order to adjust said reference plate.

5. A lapping device as defined in claim 1 further comprising spring means for biasing said substrate mounting assembly against said reference plate.

6. A lapping device as defined in claim 5 wherein said substrate mounting assembly comprises a platform operably connected to said pin means, at least three adjustable micrometers mounted in said platform and said pin means adapted to allow for the tilting of said platform when said micrometers are adjusted.

7. A lapping device as defined in claim 6 wherein said pin means comprises a plurality of pins mounted on said shaft and in inter-engaging relationship with a self-aligning ball bearing assembly operably connected between said shaft and said platform.

8. A lapping device as defined in claim 4 further comprising spring means for biasing said substrate assembly against said reference plate.

9. A lapping device as defined in claim 8 wherein said substrate mounting assembly comprises a platform operably connected to said pin means, at least three adjustable micrometers mounted in said platform and said pin means adapted to allow for the tilting of said platform when said last mentioned micrometers are adjusted.

10. A lapping device as defined in claim 9 wherein said pin means comprises a plurality of pins mounted on said shaft and in inter-engaging relationship with a self-aligning ball bearing assembly operably connected between said shaft and said platform.

References Cited UNITED STATES PATENTS 2,209,071 7/1940 Bullard 5113l 2,722,785 11/1955 Porter 51-431 2,971,298 2/1961 Garthwaite 5l131 2,979,863 4/ 1961 Emis 5l131 2,983,086 5/1961 Chapelle 51-13l HAROLD D. WHITHEAD, Primary Examiner US. Cl. X.R. 

